1. Field of the Invention
The present invention relates to a dry etching method used for a silicon thin film of a thin film transistor which is formed on an insulating film or on a semiconductor substrate, constituting a semiconductor integrated circuit and a liquid crystal display device.
2. Description of the Related Art
A thin film transistor (hereinafter, referred to as a "TFT") is widely used as a switching element in a liquid crystal display device and a semiconductor integrated circuit. An intrinsic silicon thin film (hereinafter, referred to as an "i layer") is used as a semiconductor layer constituting the TFT. A metal film is often used as a source electrode and a drain electrode which constitute the TFT. However, the interface resistance (contact resistance) between the metal film and the i layer is large. Accordingly, in the TFT an n.sup.+ silicon thin film (hereinafter referred to as an "n.sup.+ layer") having a small resistance is formed as a contact layer between the i layer and the source electrode and between the i layer and the drain electrode, respectively. In the case of forming such a TFT, an i layer and an n.sup.+ layer are successively formed and then patterned. Thus, in a step for patterning the n.sup.+ layer to form a channel section, it is required to selectively etch the n.sup.+ layer. Conventionally, examples of methods for selectively etching an n.sup.+ layer include a wet etching method and a dry etching method. The wet etching method uses a solution obtained by diluting a mixed etchant of hydrofluoric acid and nitric acid with water or acetic acid. The dry etching method uses an etching gas obtained by adding oxygen gas to mixed gas of carbon tetrachloride gas and freon-14. However, the wet etching method has raised a problem as follows: When the mixed etchant has a smaller dilution concentration, the etch rate thereof becomes higher, and stains (oxide skins) will be partially generated on a silicon thin film to be etched, degrading the controllability of etching and the uniformity in the etch rate. On the other hand, the dry etching method is superior in the controllability of etching and the uniformity of the etch rate. However, the dry etching method has also a problem in that the use of carbon tetrachloride gas for an etching gas will be prohibited in 1996 because of the freon regulations.